US4977398A

US4977398A - Color to monochrome conversion

Info

Publication number: US4977398A
Application number: US07/409,269
Authority: US
Inventors: Robert M. Pleva; Martin Randall
Original assignee: Chips and Technologies LLC
Current assignee: Intel Corp
Priority date: 1988-01-15
Filing date: 1989-09-18
Publication date: 1990-12-11
Anticipated expiration: 2008-01-15

Abstract

The present invention provides a contrast enhancing method and circuit for mapping color signals into signals for driving a display which is capable of displaying shades of gray. With the present invention a preliminary translation is first made between a foreground-background color combination and the various shades of gray that can be displayed. The contrast or separation between the foreground level of gray and the background level of gray produced by this preliminary translation is then compared to parameters set by the operator. If the preliminary translation provides the desired degree of separation between the foreground and the background gray levels, no further translation takes place; however, if the desired degree of separaton was not achieved by the preliminary translation, the system forces further separation between the foreground and background by taking the following action: (a) if the background was darker than the foreground, the background is made still darker and/or the foreground is made still lighter (b) if the background was lighter than the foreground the background is made still lighter and/or the foreground is made still darker.

Description

This is a continuation of application Ser. No. 144,849, filed Jan. 15, 1988, now abandoned.

FIELD OF THE INVENTION

The present invention relates to personal computers and more particularly to method and system for driving a personal computer display that displays shades of gray.

BACKGROUND AND PRIOR ART

Several different types of displays are commonly used with commercially available personal computers. Such displays can be categorized as either monochrome displays or color displays. Some monochrome displays can display shades of gray.

A display is generally connected to a personal computer through a display adapter. There are many different types of display adapters commercially available; however, most of the commercially available display adapters operate in accordance with one of the defacto standards that exist in this area. Among the defacto standards are standards termed Color Graphics Adapter (generally referred to as CGA), the Enhanced Graphics Adapter (generally referred to as ECA), and the Video Graphics Array (generally referred to as VGA). CGA, VGA, and EGA adapters are widely available from a variety of sources. Each of these adapters has a character mode.

When a video adapter is operating in character mode, there are color control bits associated with each data byte. For example, when a CGA is operating in character mode, each data byte has eight color control bits associated therewith. Four of the color control bits control the background color and four of the color control bits control the foreground color. The eight control bits give sixteen possible foreground colors and sixteen possible background colors for the associated character. FIG. 1A shows a CGA 10A driving a color display 11A. In a standard manner (not explicitly shown) the CGA 10A generates (a) a four bit background color control signal designated BGC (b) a four bit foreground color control signal designated FGC and (c) a one bit background or foreground select signal designated BG/FG. The signals BGC, FGC, and BG/FG are provided to a selector 10As which in turn generates a four bit signal 13A which together with timing signals 12A drive the display 11A. As the CRT beam in display 11A scans the various bit positions on the face of the display, the signals 13A indicate the color that should be displayed at each position. Timing signal 12A insures that the signals on line 13A are in synchronization with the scanning of the appropriate bit positions.

Many widely available publications explain the operation of the color control bits in a conventional CGA adapter and the operation of the system shown in FIG. 1A will not be explained further herein. For more information see a book entitled Inside the IBM PC by Peter Norton which was published by Prentice Hall, 1986 or see a publication entitled "Options and Adapters Technical Reference" publication S229-9612-00 part number 6322509, commercially available from the IBM Corporation.

It is well known that a Color Graphic Adapter, that is, a CGA adapter, operating in character mode can be used to drive a monochrome display. This can be done in a number of different ways. FIG. 1B illustrates one prior art technique for driving a monochrome display 11B from a CGA adapter 10B. Monochrome display 11B is driven by a one bit on-off signal 14B which indicates if each pixel on the face of the display should be "on" or "off". In normal operation of a monochrome display the "on" and "off" duty cycles of signal 14B are fixed. CGA 10B generates a four bit color control signals 138 that is identical to signal 13A in FIG. 1A. With the system shown in FIG. 1B a mapping circuit 15B translates each of the four bit signals on line 13A into a different duty cycle for the signals on line 14B. Thus one color combination on line 13A would generate a long "on" signal on line 14B, whereas a different color combination on line 13B would generate a short signal "on" signal on line 14B. Changing the duty cycle of the signal on line 14B has the effect of producing gray scale signals on display 11B.

FIG. 1C shows an example of a prior art system where a CGA drives a Monochrome LCD Display. In the prior art system shown in FIG. 1C, the eight bits of the BGC and FGC signals are mapped into a single bit foreground color signal FG and a background color signal BG. Circuit 10Cs then selects either signal FG or BG in response to signal BG/FG. A single bit signal then goes to LCD display 11C. This type of circuit is, for example, used in a personal computer sold by IBM Corporation under the trademark "PC CONVERTIBLE". For an explanation of how signals are mapped from color to monochrome in the IBM PC Convertible, see a publication commercially available from the IBM Corporation entitled PC Convertible, Technical Reference publication number SA 23-1047, page 2-47 et seq.

As indicated above, in systems where a CGA adapter is used to drive a monochrome display, the sixteen possible foreground colors and the sixteen possible background colors generated by the CGA must be mapped into foreground and background signals available with the monochrome display. For example, in the simplest situation the sixteen foreground and the sixteen background color possibilities from the CGA must be mapped into four "on" and "off" foreground-background possibilities available on a simple monochrome display. If the monochrome display can show different intensities or shades of gray, the number of possibilities on the display may be larger than four; however, there still must be a mapping of color into intensity or gray scale signals.

In the prior art, the mapping from the color signals generated by a video adapter into the signals adapted to drive a monochrome display is done in a manner that is fixed by the design of the machine. Such systems are not very flexible and they do not take into account different user preferences. Furthermore, in order to map all possible foreground-background combinations into a wide variety of gray scale combinations, a substantial amount of logic must be used, hence, in most prior art systems, all possible color combinations are mapped into only a few gray scale values.

SUMMARY OF THE INVENTION

The present invention provides a contrast enhancing method and circuit for mapping color signals into signals for driving a display which is capable of displaying shades or levels of gray.

With the present invention a preliminary translation is first made between a foreground-background color combination and levels of gray that can be displayed. The contrast or separation between the foreground level of gray and the background level of gray produced by this preliminary translation is then compared to parameters set by the operator.

If the preliminary translation provides the desired degree of separation between the foreground and the background gray levels, no further translation takes place; however, if the desired degree of separation was not achieved by the preliminary translation, the system forces further separation between the foreground and background by taking the following action: (a) if the background was darker than the foreground, the background is made still darker and/or the foreground is made still lighter (b) if the background was lighter than the foreground the background is made still lighter and/or the foreground is made still darker.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C show prior art systems.

FIG. 2 shows a block diagram of a preferred embodiment of the present invention.

FIG. 3 shows the gray values obtained by the initial assignment of color signals to values of gray.

FIG. 4 is a flow diagram of the algorithm that implements the method and system of the present invention.

FIG. 5 is a logic circuit block diagram of a system built in accordance with the present invention.

DETAILED DESCRIPTION

A block diagram of a preferred embodiment of the present invention is shown in FIG. 2. The system with which the present embodiment of the invention is designed to operate includes a commercially available IBM compatible personal computer (not specifically shown herein), and a commercially available LCD monochrome display 21.

The personal computer with which the present embodiment of the invention operates may be an IBM PC marketed by the IBM Corporation or it may be one of the IBM PC compatible computers that are commercially available from a wide variety of sources. The color graphics adapter 20 connects and interfaces to a personal computer (not shown herein) in a conventional manner via a standard PC bus. The interface between the personal computer and CGA 20 is explained in many commercially available publications and it is not particularly relevant to the present invention, hence, it is not explained in detail herein.

The LCD display 21 may be a commercially available LCD display such as that available with the IBM PC Convertible computer commercially marketed by IBM. It is well know that an LCD display can be made to show shades of gray by having pixels "on" during one cycle of the display and "off" during other cycles of the display. For example considering eight sequential refresh cycles of display 21, if a pixel is "on" for four cycles and "off" for four cycles, the pixel will appear gray. To eliminate flicker, the "on" and "off" cycles should alternate. If in an eight sequence cycle, a pixel is "on" for seven cycles and only "off" for one cycle, the pixel will be only slightly more gray than a pixel that is "on" for the entire eight cycles. The sequencing of pixels during sequential cycles is performed by circuit 20s which accepts the mapped signals MFG and MBG and a frame count signal FC and appropriately cycles the pixels "on" and "off" during sequential frames to generate shades of gray. It has been found that if an eight cycle period is considered, putting pixels "on" from between 0 to 8 of the cycles produces eight shades of gray as shown in FIG. 3. The signals MFG and MBG are used as numerical values for indicating how many cycles, that a pixel should be "on" and how many cycles the pixel should be "off". Given the numeric values in the MFG and MBG signals and the frame count signal (which can be generated by a simple counter), the circuitry in modulator circuit 20s sequences the pixel signals "on" and "off" over an appropriate number of frames. Circuit 20s also does the background-foreground selection in response to signal BG/FG. The operation and structure of circuit 20s is conventional and will not be explained further.

The present embodiment of the invention is directed to the CGA circuit 20 and more particularly to the parts of CGA 20 that translates the four background color bits UMBG and the four foreground color bits UMFG into the mapped signals MBG and MFG. The physical structure of the circuit 20 is not relevant to the present invention. What is significant is the logic performed. Circuit 20 could be physically implemented using any number of well known techniques such as by using a PLA, by using a programmed microprocessor or by using a custom logic integrated circuit chip.

As previously explained, the CGA adapter 20 when it is operating in text mode (which is the only mode of operation relevant to the present invention) generates signals that have eight color bits associated with each byte of data. Four of the color bits indicate the background color and four of the color bits indicate the foreground color. In FIG. 2, these signals are designated UMBG and UMFB (designating unmapped background color and unmapped foreground color).

The four color bits associated with the foreground and the four color bits associated with the background indicate whether the colors BLUE, GREEN, and RED, and the INTENSITY should be "on" or "off" when the foreground or the background of the associated byte of data is displayed. The four color bits provide sixteen combinations for the foreground color and sixteen combinations for the background color.

As a first step in the process of the present invention (a) the four bits that indicate foreground color are assigned a gray scale value and (b) the four bits that indicate the background color are assigned to a gray scale value. FIG. 3 shows the translation table used for the initial assignment of color designations to gray scale values. Thus as a first step, the present invention generates a particular foreground-background color combination or gray scale value in accordance with a fixed translation table. The translation table is shown in FIG. 3. Next the system determines if the initial assignment meets certain criteria. If the initial assignment does not meet the established criteria, the assignment is changed so that the criteria can, in so far as possible be met.

The initial assignment of gray scale values to the RED, GREEN, RED and INTENSITY bits is done by giving each of the color signals a binary value as shown in FIG. 3. The chart shown in FIG. 3 has the INTENSITY bit as the lowest binary value and the RED bit as the highest binary value, with the BLUE and GREEN bits having the second and third highest binary values. The assignment of numeric values to particular color bits is an arbitrary assignment and the embodiment would work with bits being given different numeric values. (the high order bit NF3 is used elsewhere as will be explained later and if the assignment where changed, one would use the high order bit of the new assignment for the NF3 signal). It is important to note that the color signals are assigned numeric values and that subsequent arithmetic operations are performed on these numeric values in order to enhance the contrast.

The criteria against which the results of the initial assignment are compared is established by a number of parameters that are set by the operator. The parameters set by the operator are the following:

______________________________________                                    
Kth   Threshold constant: This is a parameter that can be                 
      set by the operator to indicate a minimal                           
      acceptable separation between the foreground and                    
      background gray scale levels.                                       
Kfg   Foreground Shift Constant: This is a parameter that                 
      can be set by the operator to indicate the amount                   
      that the foreground should be shifted if Kth is                     
      not met.                                                            
Kbg   Background Shift Constant: This is a parameter that                 
      can be set by the operator to indicate the amount                   
      that the background should be shifted if Kth is                     
      not met.                                                            
SAT   Saturation Control: This is a parameter set by the                  
      operator to indicate the type of arithmetic that                    
      should be used when numbers are being added. The                    
      differences that occur depending on the setting of                  
      this parameter will be explained later.                             
______________________________________

The steps used to insure that an initial assignment meets the established criteria is shown in FIG. 4. FIG. 4 also shows how the initial assignment is corrected if the results of the initial assignment do not meet the established criteria.

The following abbreviations are used in FIG. 4.

______________________________________                                    
UMFG   This is the unmapped foreground color signal that is               
       generated in a conventional manner and provided by                 
       color graphics adapter 20 to mapping circuit 20 m.                 
UMBG   This is the unmapped background color signal that is               
       generated in a conventional manner and provided                    
       by color graphics adapter 20 to mapping circuit                    
       20 m.                                                              
MFG    This is the mapped foreground color signal that is                 
       generated by mapping circuit 20 m.                                 
MBG    This the mapped background color signal that is                    
       generated by mapping circuit 20 m.                                 
______________________________________

Color Graphics adapter 20 generates four bit color control signals UMFG and UMBG in a conventional manner (not explained in detail herein). Signals UMFG and UMBG each have four bits representing RED, GREEN, BLUE and INTENSITY. Each time the color graphics adapter 20 generates a new eight bit color signal the algorithm (shown in FIG. 4) is performed.

One special case that is specifically accommodated is what is termed "convert to binary". This means that the parameters are set so that the display does not show shades of gray and all colors are converted to black or white. The operator indicates that he wants a conversion to binary by setting the control parameters as follows:

______________________________________                                    
Kth to 15                                                                 
       (which is the maximum allowable value of Kth)                      
Kfg to 15                                                                 
       (which is the maximum allowable value of Kfg)                      
Kbg to 15                                                                 
       (which is the maximum allowable value of Kbg)                      
Sat to 1                                                                  
       (which means allow saturation and no wrapping)                     
______________________________________

As will be seen with the parameters set as shown above all colors are converted to either black or white. The algorithm used to test and if necessary modify the initial mapping is shown in FIG. 4 The algorithm operates as follows:

Block 30: The delta (D) between UMFG and UMBG as shown by the assignment table in FIG. 3 is calculated. If the calculated delta is larger than the parameter Kth, no translation is necessary and (a) MFG is set to UMFG and MBG is set to UMBG.

Block 31: If the delta between UMFG and UMBG equals zero, this indicates that the background color equals the foreground color and special action is taken as will be explained later with reference to

blocks

33 and 34.

Block 32: A determination is made as to whether the background or the foreground is lighter. If UMFG is less than UMBG, the difference between the foreground and the background is increased by setting MFG to the value of UMFG less the parameter Kfg and setting MBG to the value of UMBG plus the parameter Kbg. If UMFG is larger than UMBG, the difference between the foreground and the background is increased by setting MFG to the value of UMFG plus the parameter Kfg and setting Mbg to the value of UMBG minus the parameter KBG.

BLOCK 33: This is a check for the special case where the operator has indicated that conversion to binary is desired. If Kfg and Kbg do not both equal 15, it means that conversion to binary is not desired. Where conversion to binary is not desired and the foreground color equals the background color, no shifting is performed. MFG is set equal to UMFG and MBG is set equal to UMBG.

BLOCK 34: This block comes into operation only in the case where the background equals the foreground and the operator has indicated that conversion to binary is desired. Where this special case is present, the value of the high order bit in the UMFG color is interrogated to determine if the unmapped signals are in the top or bottom half of the table shown in FIG. 3. Depending upon whether the signals are above or below the middle of the table, both the foreground and background colors are incremented or decremented in the same direction.

A specific embodiment of hardware that implements the algorithm shown in FIG. 4 is shown in FIG. 5. It should be clearly understood that many other implementations are possible including the possibility of implementing the algorithm completely in software.

The hardware shown in FIG. 5 requires a number of control signals. These control signals are generated by conventional combinatorial logic (not specifically shown) according to the Boolean equations given below.

The first step in practicing the present invention is the unmapped foreground signal UMFG and unmapped background signal UMBG must be assigned gray scale vales according to the table shown in FIG. 3. This assignment is done by merely assigning binary values to the RED, GREEN, BLUE and INTENSITY bits in the four bit color control signals generated n a conventional manner by the CGA 20. In the present embodiment, the bits are ordered as shown in FIG. 3. This for example means that the bit representing RED is assigned as the high order binary bit, the bit representing GREEN is assigned the next binary position, the bit representing BLUE is assigned the next binary position and the INTENSITY bit is assigned the low order binary position. This assignment is shown by

blocks

50 and 51 in FIG. 5. It should be understood that in the simple case described here, blocks 50 and 51 merely in essence represent an ordering of the wires in the cables; however, a more complex initial mapping could be performed. The input is the conventional RED (R), BLUE (B), GREEN (G), and INTENSITY (I) bits, and the output is a numeric signal with these bits in a numerically ordered position.

The symbols used in the equations given below represent signals that have the following meaning.

M a signal generated by comparator 57 which indicates that the values determined in the initial assignment need be modified.

FM a signal generated by logic 52 which indicates that the foreground color assignment is at the maximum value.

BM a signal generated by logic 58 which indicates that the background color assignment is at the maximum value.

FEB a signal generated by logic 55 which indicates that the foreground color equals the background.

FGB a signal generated by logic 55 which indicates that the foreground value is larger than the background value.

NFEB a signal generated by logic which indicates that the foreground color does not equal the background color.

NFGB a signal generated by logic which indicates that the foreground color is not greater than the background color.

NF3 a signal which indicates that the high order bit of the IFG signal is not "on".

The signals required by the logic in FIG. 5 are as follows:

______________________________________                                    
A signal designated PASS which is equal to:                               
[M and NFEB] or [M and FM and BM ]                                        
A signal designated FGSUBTRACT which is equal to:                         
[M and NFEB and NFGB] or [FEB and FM and BM and NF3]                      
A signal designated BGSUBTRACT which is equal to:                         
[M and NFEB and FGB] or [FEB and FM and BM and NF3]                       
______________________________________

The logic in FIG. 5 has four main part designated A, B, C, and D. Part A consists of

blocks

50 and 51 and it does the initial assignment of gray values in accordance with the table shown in FIG. 3. The unmapped foreground and background color signals when they are ordered in accordance with the chart in FIG. 3 are designated IAFG and IABG respectively to indicate that they are the initially assigned foreground and background signals.

Blocks

50 and 51 essentially represent a plug board where the bits representing RED, GREEN, BLUE, and INTENSITY are assigned binary values as shown in FIG. 3. This assignment is entirely arbitrary.

Part B which consists of

blocks

52, 53 and 54 performs the modification of the foreground signal when appropriate. Depending on whether the PASS signal is active, logic 52 either passes Kfg or zero to block 53. Block 52 also generates signal FM according to the previously given definition. Block 53 in turn passes either its input or the complement thereof to block 54 depending on whether signal FGSUBTRACT is true. Finally block 54 does the appropriate addition to generate the MFG signal. Signal FGSUBTRACT goes to block 54 in order to enable the logic to correctly handle the overflow condition.

Part C which consists of

blocks

55, 56 and 57 determines if the initial assignment needs to be changed. If the initial mapping needs to be changed, the M signal is generated by block 57. Block 55 is conventional logic that generates signals NFEB, FEB, and FGB in accordance with the previously given definitions. Block 56 subtracts the signals IABG and IAFG. Which signal is subtracted from which signal is determined by the value of signal FGB which indicates if the foreground is larger than the background. The output from block 56 is always positive and it represents the numerical difference between the initially assigned foreground and background signals. Block 57 compares the output of block 56 to the Kth signal to determine if the numerical difference (i.e. the delta) is larger than the threshold signal. If the numerical difference is larger than the threshold signal, the M signal is generated.

Part D which consists of

blocks

58, 59 and 60 performs the modification of the background signal when appropriate. Depending on whether the PASS signal is active, logic 58 either passes Kbg or zero to block 59. Block 58 also generates the signal BM according to the previously given definition. Block 59 in turn passes either its input or the complement thereof to block 60 depending on whether signal BGSUBTRACT is present. Finally block 60 does the appropriate addition to generate the MBG signal. Signal BGSUBTRACT goes to block 60 in order to enable the logic to correctly handle the overflow condition.

Circuits

53 and 59 together with

circuits

54 and 60 in effect perform conventional saturating or non saturating addition or subtraction depending on the value of signal SAT. With respect to the chart in FIG. 3, saturating addition or subtraction means that when a number reaches the top or bottom of the chart, it stays at the maximum or minimum value. Non saturating addition means that wrap around occurs, that is when the sum of two numbers goes past the top or bottom of the chart in FIG. 3, the values wrap around to the other end of the chart.

The choice of value for the parameter SAT will depend upon the particular application of the system and upon the preference of the operator. The important point is that the system allows the operator this option.

The invention has been shown herein as applied to color signals generated by a CGA adapter. It should be understood that the invention is equally applicable to other adapters such as VGA and EGA adapters each of which produce color signals. With the present invention the color signals produced by any device can be assigned numeric values representing shades of gray. These values can be manipulated arithmetically and tested against criteria such as set points established by the operator. If the values do not satisfy the set criteria, the values are manipulated as arithmetic quantities in order to enhance the contrast.

While a particular embodiment of the invention has been shown and claimed, it should be clearly understood that the invention is not limited to the specific embodiment shown herein. Many modifications may be made without departing from the spirit and scope of the invention. The description of a specific embodiment is not meant to limit the scope of the invention. It is contemplated and specifically stated that the applicant's invention covers all modifications and alternatives to the specific embodiment shown which may fall within the words and spirit of the appended claims. It is to be fully understood that the description of a preferred embodiment is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention.

Claims

I claim:

1. A contrast enhancing circuit for mapping color signals into signals for driving a display which is capable of displaying levels of gray comprising:

means for making an initial assignment of foreground and background colors to levels of gray;

means for determining if the separation between the foreground gray level and the background gray level produced by said initial assignment exceeds a predetermined separation threshold; and

means for increasing said separation when said threshold is not exceeded by said separation by modifying said initially assigned levels of gray of said foreground and background colors without modifying levels of gray for alternate foreground and background colors which exceed said separation.

2. The circuit recited in claim 1 including means for increasing said separation by making the background still darker if the background is darker than the foreground.

3. The circuit of claim 2 wherein the means for making an initial assignment includes means for making the initial assignment for the background and foreground colors from a single range of possible gray levels.

4. The circuit recited in claim 1 including means for increasing said separation by making the foreground still lighter if the foreground is lighter than the background.

5. The circuit of claim 4 wherein the means for making an initial assignment includes means for making the initial assignment for the background and foreground colors from a single range of possible gray levels.

6. The circuit of claim 1 wherein the means for making an initial assignment includes means for making the initial assignment for the background and foreground colors from a single range of possible gray levels.

7. A contrast enhancing method for mapping color signals into signals for driving a display which is capable of displaying levels of gray comprising the steps of:

making an initial assignment of foreground and background colors to levels of gray;

determining if the separation between the foreground level of gray and the background level of gray produced by said initial assignment exceeds a predetermined separation threshold; and

increasing said separation when said threshold is not exceeded by said separation by modifying said initially assigned values without modifying values for levels of gray for alternate foreground and background colors which exceed said separation.

8. The method recited in claim 7 where said separation is increased by making the background still darker if the background is darker than the foreground.

9. The method of claim 8 wherein the step of making an initial assignment further includes the step of assigning the levels of gray to the foreground and to the background from a single range of gray scale levels.

10. The method of claim 7 where said separation is increased by making the foreground still lighter if the foreground is lighter than the background.

11. The method of claim 10 wherein the step of making an initial assignment further includes the step of assigning the levels of gray to the foreground and to the background from a single range of gray scale levels.

12. The method of claim 7 wherein the step of making an initial assignment further includes the step of assigning the levels of gray to the foreground and to the background from a single range of gray scale levels.

13. A system for translating a plurality of color signals into a plurality of signals for driving a display which is capable of displaying shades of gray, comprising:

means for making an initial assignment of numeric values to said color signals, said numeric values designating preselected shades of gray and a particular numeric value representative of a particular gray scale level;

means for testing a contrast between a gray scale level combination produced by said initial assignment to determine if said contrast meets a prespecified criteria; and

means, operative if said contrast does not meet said prespecified criteria, for arithmetically operating on said numeric values of said initial assignment in order to enhance said contrast,

said arithmetically operating means further including means for operating on only a gray scale level of said gray scale level combination to be displayed which does not meet said prespecified criteria, without altering any values for alternate gray scale level combinations which meet said prespecified criteria.

14. A contrast enhancing method for translating a plurality of color signals into a plurality of signals for driving a display which is capable of displaying shades of gray, comprising the steps of:

making an initial assignment of numeric values to said color signals, said numeric values designating preselected shades of gray and a particular numeric value representative of a particular gray scale level;

testing a contrast between a gray scale level combination produced by said initial assignment to determine if said contrast meets a prespecified criteria; and

operating arithmetically on said initial assignment if said contrast does not meet said prespecified criteria, in order to enhance said contrast,

said operating step further including the step of operating on only a gray scale level of said gray scale level combination to be displayed which does not meet said prespecified criteria, without altering any values for alternate gray scale level combinations which meet said prespecified criteria.